Strategies for facilitating functional recovery following injury to athe nervous system are often based on normal developmental processes, such as neuron proliferation, axon growth, and target selection. However, the possibility that proliferation could be reinstituted postnatally is sufficiently remote that neurons lost to injury are considered unreplaceable. While neuron proliferation is unlikely to occur postnatally, several lines of evidence support the possibility that neuron number is augmented during postnatal life by the late differentiation of immature neurons. If so, such late differentiation may provide a mens to replace neurons lost to injury without neuron proliferation. The objective of this proposal is directed toward this fundamental question of neural organization; Are neurons added to tahe nervous system in the growing and mature adult? Without some form of compensation, sensory discrimination and motor precision would be degraded as the ratio of target size to neurons increased. The proposed experiments are directed toward determining whether larger rats have more sensory neurons than small animals, whether more sensory neurons innervate particular targets in larger rats, and the mechanism whereby neurons are added. If neurons are indeed added during the normal process of maturation, then the possibility of enlisting this capability in order to ameliorate the effects of injury to the nervous system can be considered.